The present invention relates generally to wireless communication systems and, in particular, to mobile-telephone location systems.
The Federal Communication Commission (FCC) has requested all cellular and Personal Communication System (PCS) based operators to provide emergency 911 location services for mobile-telephones. Solutions to this challenging issue include typical prior art systems that use well-known location techniques, such as time difference of arrival (TDOA), ranging, angle-of-arrival, etc., to yield a good estimate of a mobile-telephone""s location. These location techniques typically involve the use of times-of-arrival observed at distributed locations together with xe2x80x9ctriangulationxe2x80x9d principles to determine a mobile-telephone""s location.
To determine a mobile-telephone""s location to an acceptable accuracy using triangulation principles, at least three detectors should be able to detect a signal transmitted from a signal source and record the times-of-arrival at each of the detectors. Alternately, to determine a mobile-telephone""s location, one detector should be able to detect a signal transmitted from at least three signal sources and record the times-of-arrival of each signal at the detector. Referring to FIG. 1, there is shown an illustration of a prior art location system 10. As shown in FIG. 1, the location system 10 includes a plurality of detectors 12-18 which are associated with geographical areas commonly referred to as cells 22-28. To locate a mobile-telephone 30, the detectors in the cells surrounding the mobile-telephone 30, i.e., detectors 12,13,15, should each be able to detect and record times-of-arrival for a signal transmitted from the mobile-telephone 30.
In some instances, a detector may detect the arrival of the same signal more than once due to reflectionsxe2x80x94that is, the signal transmitted from the mobile-telephone may travel multiple paths to the same detector. For example, suppose the mobile-telephone 30 transmits a signal S. The signal S will arrive at the detector 13 via a path P1, which is the shortest or most direct path from the mobile-telephone to the detector. The path P1 is referred to herein as a xe2x80x9cline-of-sight path.xe2x80x9d The same signal S, however, may also arrive at the same detector 13 along paths P2, P3, which are paths longer or less direct than the line-of-sight path P1. The paths P2, P3 are referred to herein as xe2x80x9cnon line-of-sight paths.xe2x80x9d It is well-known that the times-of-arrival for line-of-sight signals provide location systems with a more accurate estimation of a mobile-telephone""s location than the times-of-arrival for non line-of-sight signals, thus a location system depends in part on its ability to receive and/or detect line-of-sight signals.
Line-of-sight signals are distinguishable from non line-of-sight signals (provided they are sufficiently far apart from each other) because the former signals will travel the shortest, most direct path to the detectors, thereby arriving first at the detectors. Thus, the first signal detected by the detector is generally assume to be the line-of-sight signal. However, in areas such as urban environments, the line-of-sight signals may be highly attenuated or degraded by the time they arrive at the detector, thereby impeding the detector""s ability to detect such signals. The location system""s inability to detect line-of-sight signals may cause it to mistakenly assume later arriving non line-of-sight signals to be line-of-sight signals, thereby adversely affecting the location system""s estimate of the mobile-telephone""s location.
To compensate for possible attenuation or degradation of the line-of-sight signals, the prior art location systems may require the mobile-telephone to transmit such signal at higher power levels. This would increase the signal-to-noise ratio at the detector, thereby increasing the likelihood of detecting the line-of-sight signals. However, increasing the power level of the signals increases the interference level, i.e., noise, for other mobile-telephones in the same or neighboring cells. One possible solution for achieving increased signal-to-noise ratio without prolong interference level increases involves the transmission of signals at higher power levels for short time periods and performing extended integration in demodulation. This solution, however, has associated capacity/performance penalties, i.e., increased interference levels. Accordingly, there exist a need for a more accurate system for estimating a mobile-telephone""s location without significantly increasing the interference level for other mobile-telephones in the same or neighboring cells.
The present invention is a more accurate system for estimating the location of a mobile-telephone without significantly increasing the interference level for other mobile-telephones in the same or neighboring cells. Specifically, the present invention uses improved line-of-sight signal detection to enhance its estimation of a mobile-telephone""s location.
In one embodiment, the present invention comprises a wireless communication network and a plurality of location terminals deployed throughout a cell to increase the chances of detecting line-of-sight signals and the signal-to-noise ratio at the location terminals. In this embodiment, the wireless communication network transmits receive information to the plurality of location terminals instructing each of the location terminals to receive a signal from a particular mobile-telephone. Specifically, the location terminal is instructed to monitor a specific communication channel during a specific time interval. Upon receiving the signal, each of the location terminals produces detection information that is used to estimate a location for the mobile-telephone. Specifically, the detection information includes times-of-arrival and strengths of the signal at each of the location terminals.